The present invention relates to the purification of acetic acid. More particularly, the present invention relates to the purification of product streams of acetic acid produced using a catalytic system containing iodine and which streams contain residual iodine and water.
A process has recently been developed for the preparation of carboxylic acids, notably acetic acid, by the reaction of an alcohol or an ester, or ether and halide derivatives thereof, with carbon monoxide in contact with catalyst systems formed on mixing of a rhodium or iridium component and a halogen component which is usually an iodine component in the presence of carbon monoxide. Hydrogen iodide and/or an alkyl iodide, such as methyl iodide, are employed as the iodine component in these catalyst systems.
While acetic acid produced by the above described process is generally of relatively high purity as far as other organic by-products are concerned, it does contain water and relatively small amounts of iodine as contaminants. In order that the acid can be utilized in further reactions and other uses, it must generally be freed from any water which is present as well as from the small amounts of iodine contaminants present. When freed from such impurities the acid thus produced is admirably suited for practically all uses in commerce and industry of such acid and finds a ready market therein.
A plural-stage distillation system for removal of water as well as the removal of halogen-containing impurities, especially iodine, from streams of carboxylic acids has been described and claimed in U.S. Pat. No. 3,769,177 issued on Oct. 30, 1973. In this process, a stream of carboxylic acid such as acetic acid and containing as contaminants water and certain halogenated materials such as iodine is introduced into a distillation zone. Alkyl halides present such as methyl iodide along with a portion of the water present are removed as an overhead stream from this zone while substantially all hydrogen halides such as hydrogen iodide present are removed in the bottoms stream. A stream from the middle portion of the first distillation zone is removed from said zone and introduced into a second distillation zone wherein there is removed as an overhead stream the remaining portion of the water present. A stream from the middle portion of the second distillation zone is recycled to the first zone and a product acid stream essentially dry and substantially free of the halogen components is removed at or near the bottom of the second distillation zone. This system is generally satisfactory in that there is virtually no waste in the process, all streams being adaptable to recycle to the process for producing the acid mixture which is purified. However, certain disadvantages develop in this system when purifying acetic acid, for example, over long sustained periods of operation. It is difficult to maintain column stability in the second distillation zone. The sidestream taken from this distillation zone is taken from a plate in the middle portion of the zone so selected at a specific temperature and pressure of operation that the concentration of hydrogen iodide is near and, preferably, at the highest of the entire zone on that plate since the purpose of this recycle stream of acid and water is to remove all of the remaining hydrogen iodide present in the second distillation zone. Thus, a special technique is required for control in this zone. Control cannot be effected by the use of conventional distillation control means for monitoring overhead and/or bottoms compositions. In addition, the necessity for withdrawing a sidestream as indicated generally requires a higher temperature than would otherwise be employed which along with the high hydrogen iodide concentration creates an environment conducive to corrosion.
A method has now been discovered for conducting the two-zone distillation for the purification of acetic acid wherein the sidedraw stream from the second distillation zone which is recycled to the first distillation zone can be eliminated to overcome the disadvantages mentioned above. In addition, the new method of operation provides for savings in energy, i.e., stream requirements, and in increased capacity in the system.